Generate Open API spec from AWS API Gateway

endpoints-openapi.ts

import path from 'node:path'
import { Duration } from 'aws-cdk-lib'
import { NodejsFunction } from 'aws-cdk-lib/aws-lambda-nodejs'
import { Runtime } from 'aws-cdk-lib/aws-lambda'
import { Construct } from 'constructs'
import { PolicyStatement } from 'aws-cdk-lib/aws-iam'

export const createOpenAPILambda = (scope: Construct): NodejsFunction => {
  const apiInformationPolicy = new PolicyStatement({
    actions: ['apigateway:GET'],
    resources: ['arn:aws:apigateway:*::/restapis/*/stages/*/exports/*']
  })

  const lambda = new NodejsFunction(scope, 'OpenAPIFn', {
    memorySize: 256,
    timeout: Duration.seconds(5),
    runtime: Runtime.NODEJS_16_X,
    handler: 'handler',
    entry: path.join(__dirname, '../../routes/openapi.ts'),
    bundling: {
      minify: true,
      externalModules: ['aws-sdk']
    }
  })

  lambda.addToRolePolicy(apiInformationPolicy)

  return lambda
}

routes-openapi.ts

import {
  APIGatewayProxyEvent,
  APIGatewayProxyResult
} from 'aws-lambda/trigger/api-gateway-proxy'
import { APIGatewayClient, GetExportCommand } from '@aws-sdk/client-api-gateway'
import { lambdaResponse } from './helpers/lambdaResponse'

export const handler = async (event: APIGatewayProxyEvent): Promise<APIGatewayProxyResult> => {
  let openapiSpec, error
  try {
    const client = new APIGatewayClient({
      region: 'eu-west-1'
    })
    const command = new GetExportCommand({
      restApiId: event.requestContext.apiId,
      stageName: event.requestContext.stage,
      exportType: 'oas30'
    })
    const commandResponse = await client.send(command)
    // GetExportCommandOutput: https://docs.aws.amazon.com/AWSJavaScriptSDK/v3/latest/clients/client-api-gateway/interfaces/getexportcommandoutput.html
    const decodedBody = new TextDecoder().decode(commandResponse.body)
    openapiSpec = JSON.parse(decodedBody)
    console.log('Successfully generated Open API spec')
  } catch (ex) {
    error = (ex as Error).message
    console.log('Unable to generate Open API spec, error:', error)
  }

  const responseData = openapiSpec ?? { message: 'Unable to retrieve or decode OpenAPI Spec', openapiSpec, error, event }
  return lambdaResponse(200, JSON.stringify(responseData))
}